1. Field of the Invention:
The present invention relates to thermoplastic polyurethanes, polyurethane elastic fibers therefrom, and a method for producing the fibers. The thermoplastic polyurethanes of the invention give polyurethane fibers having good heat resistance, good hot water resistance and good elastic recovery.
2. Description of the Prior Art:
For producing polyurethane elastic fibers, known are methods of dry spinning, wetspinning, melt spinning, etc. Of those, polyurethane elastic fibers as obtained according to a melt-spinning method have good thermosettability, abrasion resistance and transparency, and, in addition, the production costs for them are low. Therefore, the consumption thereof is much increasing in recent days. However, as compared with polyurethane fibers as obtained according to a dry-spinning method, those as obtained according to a melt-spinning method are difficult to form stiff hard segments, and, therefore, their heat resistance and hot water resistance are not satisfactory.
For these reasons, heretofore, various methods have been proposed for improving the heat resistance and the hot water resistance of melt-spun, polyurethane elastic fibers. One conventional method comprises forming an intermolecular crosslinked structure of polyurethanes that constitute fibers. For example, JP-A-48-58095, JP-B-50-10630 and JP-A-6-294012 disclose a technique of forming a crosslinked structure in the hard segment parts of polyurethanes by the use of a trifunctional or higher polyfunctional chain extender such as trimethylolpropane. However, the heat resistance of the conventional polyurethanes having such a crosslinked structure as formed in the hard segment parts is not still satisfactory, and, therefore, the heat resistance of the polyurethane elastic fibers to be obtained from them is not also satisfactory.
Apart from the conventional method noted above, another method has been proposed for obtaining polyurethane elastic fibers having reduced residual strain and increased dynamic resiliency, which comprises preparing a polyurethane by reacting a hydroxy-polyester (this is prepared by reacting a glycol having a secondary hydroxyl group and a trifunctional or higher polyalcohol with a dicarboxylic acid) and a chain extender with an organic diisocyanate, followed by melt-spinning the resulting polyurethane into polyurethane elastic fibers (see JP-B-42-3958). However, the heat resistance of the polyurethane elastic fibers as obtained according to this method is still poor, and the method can not attain the object of obtaining polyurethane elastic fibers having good heat resistance. In addition, having actually produced polyurethanes and polyurethane elastic fibers according to the method of the examples described in JP-B-42-3958, we, the present inventors have found that the fibers are inferior in not only the heat resistance but also the hot water resistance and other properties.
JP-B-42-5251 has proposed a method for producing polyurethane elastic fibers, which comprises preparing a polyurethane from a polyol having more than two functional hydroxyl groups followed by spinning it in a wet chemical spinning manner. However, the wet chemical spinning in the proposed method often gives fibers that are poorly homogeneous and have poor abrasion resistance. On the other hand, in JP-A-59-179513 and 63-159519, it is disclosed that polyurethanes as produced from prepolymers comprising soft segments from a mixture of a polyester-diol and a polyether-diol have good solution stability during their spinning. However, the heat resistance and the hot water resistance of the polyurethanes disclosed therein are not satisfactory.
Given that situation, JP-A-3-220311 discloses polyurethanes as produced from a polyester-diol (this is prepared through reaction of a diol containing 3-methyl-1,5-pentanediol, and an aliphatic dicarboxylic acid component having from 6 to 12 carbon atoms), an organic diisocyanate and a chain extender, and fibers from the resulting polyurethanes. They say that the polyurethane elastic fibers have good chlorine resistance, water-proofness, fungus-proofness, elastic recovery, heat resistance, hot water resistance and elongation. Further, JP-A-9-49120 discloses polyurethane elastic fibers as produced from polyurethanes which are prepared from a polyester-polyol, an organic diisocyanate and a chain extender. They say that, when the composition of the starting component, polyester-polyol for the polyurethanes is specifically defined, the uniformity of the polyurethane fibers could be improved while the fibers preserve their good properties such as those mentioned above. The heat resistance and the hot water resistance of those polyurethane elastic fibers could be improved in some degree. However, it is still desired to further improve the properties of polyurethane elastic fibers.